1. Field of the Invention
The present invention relates to an optical information recording medium for recording/reproducing a large volumetric information signal by means of a laser light, and a method for producing the same.
2. Description of the Related Art
As optical information recording mediums capable of recording/reproducing a signal by means of a laser light, there exist a phase change type optical disc, a magnetic optical disc, a dye disc and the like. Among those discs, in the phase change type optical disc which is capable of recording/erasing a signal, a typical material to be used for a recording layer is chalcogenide. Generally, the recording layer is regarded as an unrecorded state when the material for the recording layer is in a crystal state. A signal is recorded on the recording layer such that the recording layer material is irradiated with a laser light to be melted and quenched so as to come into an amorphous state. On the other hand, in the case of erasing a signal, the recording layer material is irradiated with a smaller power laser light than at the time of recording, so as to come into a crystal state. Since the recording layer comprising chalcogenide is formed while in the amorphous state, it is necessary to previously crystallize the whole surface of the recording region to obtain an unrecorded state. Such crystallization of the whole surface of the recording region is called initialization.
As a technique for realizing high density of the recordable/erasable phase change type optical disc, it has been proposed to use a blue laser light having a wave length of about 410 mm as a light source for recording/reproduction in place of a conventionally commonly used red laser light, to increase the numerical aperture of an objective lens of an optical system, which applies the laser light for recording/reproduction onto the optical disc, from the conventional commonly used numerical aperture of 0.60 to about 0.85, thereby reducing the size of a laser light spot. It has also been proposed that, in the case of using the objective lens with the numerical aperture thereof made as large as 0.85, a transparent protective substrate on the laser light incident side have a smaller thickness, such as 0.1 mm, than the 0.6 mm thickness of the substrate of an already commercialized DVD-RAM, for the purpose of securing tilt tolerance of an optical information recording medium in terms of recording/reproduction characteristics (e.g., cf. Japanese Patent Laid-Open Publication No. 10-154351).
Further, as a technique for multiplying a recording capacity per face, a medium with a single-sided multi-layered configuration has been proposed (e.g., cf. Japanese Patent Laid-Open Publication No. 2000-036130). Moreover, a technique concerning production of a substrate of an optical disc having a guide groove on the surface thereof is also known (e.g., cf. Japanese Patent Laid-Open Publication No. 09-320100).
However, the experiments performed by the present inventors revealed the following. In the single-sided multi-layered phase change optical disc which performs recording/reproduction by means of a blue laser light, on the information layer on the side closer to the laser light source, a noise level of the disc after recording especially in the outer perimeter region degrades as compared with the inner and the middle perimeter regions. The cross section of this disc was observed with a transmission electron microscope, to find that the guide groove itself has an almost symmetric shape with respect to the center thereof over the whole region from the inner perimeter to the outer perimeter of the disc, thus having no particular problem. However, in the outer perimeter region of the disc, there was recognized a remarkable difference, e.g., about 20%, in thickness of the information layer between an inclined face portion on the inner perimeter side and an inclined face on the outer perimeter side of the guide groove.
The cause-effect relationship between the difference in layer thickness and the high noise level can be explained by considering a thermal aspect and an optical aspect as described below. First, when the layer thicknesses are different between the inclined face portion on the inner perimeter side and the inclined face portion on the outer perimeter side of the guide groove, the respective heat capacities are different, to cause the respective diffusion rates to be different. This leads to formation of a mark shape dissymmetric and distorted with respect to the track center, thereby increasing the noise level. Further, optical characteristics such as a reflectance with respect to the laser light become different between the inclined face on the inner perimeter side and the inclined face on the outer perimeter side of the guide groove, and the spot center of the laser light is thus displaced from the track center, causing a normal tracking operation to be ceased. As a result of performing a recording operation in such a state, the noise level increases.
Further, recording mediums having been put to practical use so far are not multi-layered recording mediums but recording mediums having only a single information layer. Hence in those recording mediums, a laser light is not required to be transmitted through the information layer, and for example, the reflective layer and the like are made sufficiently thick to such a degree that a transmittance thereof is almost zero. In such a state, even when the layer thickness is different between the inclined face portion on the inner perimeter side and the inclined face portion on the outer perimeter side of the guide groove, a difference in thermal or optical characteristics therebetween is considered as being in an ignorable level. As opposed to this, the information layer on the nearer side from the laser source in the multi-layered recording medium is required to have a constant transmittance of not smaller than 30%, for example, and the thermal or optical characteristics thereof are considered to be very sensitive to the difference in layer thickness, unlike the recording medium having a single information layer or the information layer on the farther side from the laser light source.
Almost all those skilled in the art form a thin layer constituting an information layer by placing a substrate and a target so as to be mutually opposed and then sputtering is performed, and the present inventors also form the thin layer in this manner. It is difficult to employ other methods for the layer formation from the perspective of convenience and practicability of a layer forming device and a layer forming rate. Generally, since recording by means of a laser light is performed by forming a mark in the guide groove and/or a flat portion between the guide grooves, the layer forming device is designed so as to keep the layer thickness uniform in the flat portion from the inner perimeter region to the outer perimeter region of the disc. However, as for the inclined face of the guide groove, an angle and a rate of particles flying by sputtering from a target as a raw material of the layer are not uniform from the inner perimeter region to the outer perimeter region of the disc. This causes some portions of the inclined face on the inner perimeter side and the inclined face on the outer perimeter side of the disc to have different thicknesses by the shadowing effect. However, since no mark is formed on the inclined face of the guide groove, and the inclined face has thus been considered to exert no direct influence on the recording/reproduction characteristics, an attempt has not been made to keep the layer thickness uniform with high accuracy between the inclined face portion on the inner perimeter side and the inclined face portion on the outer perimeter side of the guide groove over the whole region from the inner perimeter region to the outer perimeter region of the disc.
Accordingly, it is an object of the present invention to solve the above-mentioned problems and provide an optical information recording medium with favorable signal quality even in high density recording using a translucent information layer.